In electrophotography, a latent image is created on the surface of an image member which is a photoconducting material by first uniformly charging the surface and selectively exposing areas of the surface to light. A difference in electrostatic charge density is created between those areas on the surface which are exposed to light and those areas on the surface which are not exposed to light. The latent electrostatic image is developed into a visible image by electrostatic toners. The toners are selectively attracted to either the exposed or unexposed portions of the photoconductor surface, depending on the relative electrostatic charges on the photoconductor surface, the development electrode and the toner. Electrophotographic photoconductors may be a single layer or a laminate formed from two or more layers (multi-layer type and configuration). Typically, a dual layer eiectrophotographic photoconductor comprises a substrate such as a metal ground plane member on which a charge generation layer (CGL) and a charge transport layer (CTL) are coated. The charge transport layer contains a charge transport material which comprises a hole transport material or an electron transport material. For simplicity, the following discussions herein are directed to the use of a charge transport layer which comprises a hole transport material as a charge transport compound. One skilled in the art will appreciate that if the charge transport layer contains an electron transport material rather than the hole transport material, the charge placed on the photoconductor surface will be opposite that described herein.
When the charge transport layer containing a hole transport material is formed on the charge generation layer, a negative charge is typically placed on the photoconductor surface. Conversely, when the charge generation layer is formed on the charge transport layer, a positive charge is typically placed on the photoconductor surface Conventionally, the charge generation layer comprises a charge generation compound or molecule alone and/or in combination with a binder. A charge transport layer typically comprises a polymeric binder containing the charge transport compound or molecule The charge generation compounds within the charge generation layer are sensitive to image-forming radiation and photogenerate electron hole pairs therein as a result of absorbing such radiation. The charge transport layer is usually non-absorbent of the image-forming radiation and the charge transport compounds serve to transport holes to the surface of the negatively charged photoconductors. Photoconductors of this type are disclosed in the Adley et al U.S. Pat. No 5,130,215 and the Balthis et al U.S. Pat. No. 5,545,499.
Typically, the charge generation layer comprises a pigment or dye (phthalocyanines, azo compounds, squaraines, etc.), with or without a polymeric binder. Since the pigment or dye in the charge generation layer typically does not have the capability of binding or adhering effectively to a metal substrate, the polymer binder is usually inert to the electrophotographic process, but forms a stable dispersion with the pigment/dye and has good adhesive properties to the metal substrate. The electrical sensitivity associated with the charge generation layer can be affected by the nature of polymeric binder used. The polymeric binder, while forming a good dispersion, should have a greater interaction with the metal substrate rather than the pigment.
Similarly, the charge transport layer typically consists of a charge transport molecule (CTM), typically selected from arylamines, hydrazones, stilbenes, pyrazolines, and other known in the art in a polymeric binder The polymeric binder is typically a polycarbonate such as polycarbonate-A, polycarbonate-Z, etc. which provides good mechanical properties to the photoconductor. Photoconductors of this type are disclosed in the Kemmesat et al U.S. Pat. No. 6,001,523.
The photoconductor (conventionally in drum, web or belt form) is often subjected to several modes of abrasion by paper, cleaner, toner, end-seals, and the like. Therefore, it is imperative that the wear on the photoconductor be minimal for the photoconductor to have an extended long life in a printer cartridge. Increased wear on a photoconductor surface may lead to arcing of the charge roll, increased fatigue, scratches on the paper area, delamination, and the like, resulting in defects and decreased photoconductor life in the cartridge.
One approach for reducing photoconductor wear is the addition of materials to the photoconductor formulation that will either reduce the friction between the photoconductor and the other parts of the electrophotographic engine; increase the hardness of the formulation to enhance its wear resistance; or both. The use of silicon microspheres in the charge transport layer has been found to effectively reduce wear in photoconductors. Photoconductors of this type are disclosed in the Hinch et al U.S. Pat. No. 5,994,014. The use of polycarbonate-Z has also been known to exhibit improved wear resistance over polycarbonate-A. In addition, the use of polymeric blends, overcoats, organic additives (fluoropolymers, silicone oils, etc.), and inorganic additives have been known to improve the wear on the photoconductor surface. These approaches have varying effects on photoelectric properties of the photoconductors.
It may also be desirable to improve the sensitivity of the CGL in a photoconductor. Sensitivity may be improved by the use of certain pigments (e.g. Type-IV titanyt phthalocyanine instead of Type-I titanyl phthalocyanine or squarylium pigment), increasing the pigment concentration with respect to the polymeric binder, or through the use of polymeric blends in the charge generation layer.
As such, there is a continuing need for photoconductors exhibiting increased photoconductor sensitivity and enhanced resistance to wear.